Vertical niche overlap by two ocean giants with similar diets: Ocean sunfish and leatherback turtles

We used satellite tags to record the patterns of depth utilisation for four ocean sunfish (Mola mola) and two leatherback turtles (Dermochelys coriacea) moving in broadly the same area off South Africa. Individuals were tracked for between 2 and 8 months and dive data relayed via satellite. For all the sunfish and one of the turtles, we received binned data on depth distribution, while for the second turtle we received individual dive profiles along with the proportion of time spent diving. Leatherback turtles dived almost exclusively within the upper 200 m, spending only 0.6 and 0.2% of their time > 200 m. There were times when sunfish likewise occupied these relatively shallow depths. However, there were also protracted periods when sunfish spent the majority of their time much deeper, with one individual remaining around 500 m for many hours at a time. These results suggest that sunfish sometimes exploit deeply distributed prey which is beyond the foraging range of leatherback turtles. We conclude that while both species are believed to feed predominantly on gelatinous zooplankton, the fact that sunfish do not need to come to the surface to breathe means that they can occupy an expanded vertical niche compared to the leatherback turtle.     

Vertical movement and behavior of the ocean sunfish, Mola mola, in the northwest Atlantic

Pop-up satellite archival tags (PSATs) were attached to 31 ocean sunfish, Mola mola. in the Northwest Atlantic between 2005 and 2008, in order to examine their vertical movement and behavior. Tags remained attached from 7 to 242 days, with a mean attachment period of 107.2 ± 80.6 (SD) days. Fish spent greater than 30% of their time in the top 10 m of the water column, and over 80% of time in the top 200 m. The maximum depth recorded by any fish was 844 m. Temperatures experienced by tagged fish ranged from 6 to 30 °C. Vertical behavior of M. mola changed over short-term and seasonal scales. Ocean sunfish in northeastern US waters in the summer months inhabited shallower depths and spent more time at the surface than those that moved south in the winter and spring. This shift from shallow to deeper depths was especially apparent when fish entered the Gulf Stream, where they spent little time at the surface and dove to depths of 400-800 m. A diel pattern was observed in vertical behavior. Tagged fish spent more time at depth during the day and inhabited shallower waters at night. There was no observed relationship between the amount of time per day that fish spent in cold water (< 10 °C) and the amount of time fish spent near the surface (0-6 m), indicating a lack of evidence for M. mola basking at the surface as a mechanism for behavioral thermoregulation.     

First results from satellite-linked archival tagging of porbeagle shark, Lamna nasus: Area fidelity, wider-scale movements and plasticity in diel depth changes

Understanding the habitat preferences of large marine vertebrates has only recently become tractable with the widespread availability of satellite telemetry for monitoring movements and behaviour. For many species with low population abundances, however, little progress has been made in identifying space use patterns. The endothermic porbeagle shark, Lamna nasus, has declined in the North Atlantic due to severe fishing pressure, with little evidence of recovery. One potential factor exacerbating population decline is area fidelity to coastal waters where fisheries are intensive. We tested for short-term area fidelity by attaching pop-up satellite-linked archival transmitters to four porbeagles in summer 2007, resulting in 175 days total tracking time covering an estimated 10,256 km distance. Throughout July and August the sharks occupied localised areas (8,602 – 90,153 km²) within the Celtic Sea, between the south-west UK, south-west Wales and southern Ireland. Only one shark was tracked into the autumn, when it moved into deep water off the continental shelf, then north towards colder latitudes. Sharks occupied a broad vertical depth range (0 – 552 m) and water temperatures (9° - 19 °C). Dives were made frequently from the surface to near the seabed in shelf areas, however, in shelf edge habitats extended periods of time were spent at depths > 300 m. Porbeagles showed considerable plasticity in diel depth changes within and between individuals and as a function of habitat type. In addition to no obvious day-night difference in depth occupation, some sharks showed reverse diel vertical migration (DVM) (dawn ascent – dusk descent) in well-mixed coastal waters whereas normal DVM (dawn descent – dusk ascent) characterised movements into deeper, thermally well-stratified waters. The variable behaviours may reflect the need for different search strategies depending on habitat and prey types encountered. These results show porbeagles are potentially vulnerable to fisheries throughout the summer when they aggregate, and that large scale movement across national boundaries identifies the need for international conservation measures.     

Retinal topography of ganglion cells in immature ocean sunfish, <Emphasis Type="Italic">Mola mola</Emphasis>

The ocean sunfish, Mola mola, is the largest known bony fish. Based on prior studies of diet composition, it is considered to be a pelagic zooplanktivore. However, a recent study using acoustic telemetry revealed that they repeatedly dive to depths of >50 m during the day. We examined the distribution of cells within the retinal ganglion cell layer in the immature ocean sunfish (c.a. 50 cm total length) and estimated their visual acuity with respect to the main visual axis and visual fields. Visual acuity was between 3.37 and 4.41 cycles/degree. The region of highest cell density was located in the dorso-temporal retina, indicating that the main visual axis of ocean sunfish is directed towards the lower frontal portion of the visual field. This axis is considered beneficial for detecting prey items when the sunfish are migrating vertically through the water column, and in foraging behavior near the sea bottom.     

Vertical movement of dolphinfish <Emphasis Type="Italic">Coryphaena hippurus</Emphasis> as recorded by acceleration data-loggers in the northern East China Sea

Environmental changes influence foraging behavior for most animals. Dolphinfish, Coryphaena hippurus, are epipelagic predators and have a cosmopolitan tropical to warm-temperate (>20°C) distribution. We simultaneously obtained the ambient temperature and the foraging behavior (i.e., swimming speed, depth and tailbeat acceleration) of dolphinfish, using an acceleration data-logger in May, September, October, November 2007, June 2008, May and July 2010 for 8 individuals. Although the dolphinfish spent a mean ± standard deviation of 43.4 ± 27.7% of their time at the surface (0–5 m), dive excursions from the surface (DES) were observed in all individuals and maximum DES depths ranged from 50.1 to 95.4 m. DES events resulted dives below the thermocline for these dolphinfish, and there was a significantly positive relationship between the isothermal layer depth (ILD) and DES depth. Our results demonstrate that dolphinfish avoided the rapid thermal change beyond the thermocline, and their prey is most likely found in the upper layers of the thermocline. Gliding behavior during the DES phase was also observed and dolphinfish gradually descended to deeper waters with gliding. The gliding time was longer when the ILD was deeper, and fish tended to dive deeper. We suggest that dolphinfish adopt gliding behavior to search a broader range of depths for prey, while minimizing energy use.     

Habitat use and movement patterns of bull sharks Carcharhinus leucas determined using pop‐up satellite archival tags

Habitat use, movement and residency of bull sharks Carcharhinus leucas were determined using satellite pop‐up archival transmitting (PAT) tags throughout coastal areas in the U.S., Gulf of Mexico and waters off the south‐east U.S. From 2005 to 2007, 18 fish (mean size = 164 cm fork length, L_F) were tagged over all seasons. Fish retained tags for up to 85 days (median = 30 days). Based on geolocation data from initial tagging location to pop‐off location, C. leucas generally travelled c. 5–6 km day^?1 and travelled an average of 143·6 km. Overall, mean proportions of time at depth revealed C. leucas spent the majority of their time in waters <20 m. They exhibited significant differences among depths but were not found at a particular depth regardless of diurnal period. Most fish occupied temperatures c. 32° C with individuals found mostly between 26 and 33° C. Geolocation data for C. leucas were generally poor and varied considerably but tracks for two individuals revealed long distance movements. One fish travelled from the south‐east coast of the U.S. to coastal Texas near Galveston while another moved up the east coast of the U.S. to South Carolina. Data on C. leucas movements indicated that they are found primarily in shallower waters and tend to remain in the same location over long periods. While some individuals made large‐scale movements over open ocean areas, the results emphasize the importance of the coastal zone for this species as potential essential habitat, particularly in areas of high freshwater inflow.     

Movement and behaviour patterns of the critically endangered common skate Dipturus batis revealed by electronic tagging

Understanding animal distributions and habitat utilisation is vital for the management of populations, especially those of endangered species. However, this information is not available for the majority of marine species and is difficult to obtain for those with low population densities. The common skate, Dipturus batis, was once abundant and widespread in the North-East Atlantic but is now thought to be locally extinct in the Irish Sea and in the central and southern North Sea, and is listed as Critically Endangered on the IUCN Red List. The constraints of skate body morphology on locomotory mode assume low levels of activity with long periods spent resting on the seabed, therefore predicting a high degree of site fidelity. To investigate this hypothesis we tagged 8 common skate (two male and six female, mass range: 10.9–63.5 kg) with depth and temperature-logging data storage tags off the west coast of Scotland in May 2008. All 6 tags attached to females were recovered after 1–9 months at liberty. All 6 individuals exhibited pronounced site fidelity to highly localised areas. Within these local areas however, time-depth profiles were dominated by periods of high activity, with vertical movements of > 100 m being conducted on a regular, sometimes daily, basis. Intra-individual plasticity was observed in vertical activity patterns with individuals switching between low and high activity patterns. Smaller skate were generally less active and occupied deeper depths. Limited short-term horizontal movements in preferred habitats supporting apparently high foraging activity highlights the need for spatial management of ‘refugial’ populations of this once widespread fish, that appears now largely extirpated from European waters.     

Investigation of foraging habits and prey selection by humpback whales (Megaptera novaeangliae) using acoustic tags and concurrent fish surveys

Tags containing acoustic time-depth transmitters (ATDT) were attached to four humpback whales near Kodiak, Alaska. Tags allowed for whale dive depths to be recorded in real time. Acoustic and mid-water trawl surveys were conducted concurrent with tagging efforts within the study area to quantify available fish resources and describe potential prey selection by humpback whales. Recorded dives were grouped through visual assessment and t -tests. Dives that indicated likely foraging occurred at a mean maximum depth of 106.2 m with 62% of dives occurring between 92 m and 120 m. Acoustic backscatter from fish surveys was attributed to potential humpback prey based on known target strength values and 10 net tows. Capelin comprised 84% of the total potential prey abundance in the region followed by age 0 (12%) and juvenile pollock (2%), and eulachon (<1%). Although horizontally segregated in the region, both capelin and age 0 pollock were distributed at depths exceeding 92 m with maximum abundance between 107 m and 120 m. The four-tagged humpbacks were found to forage in areas with greatest capelin densities but bypassed areas of high age 0 pollock abundance. The location and diving behavior of tagged whales suggested that whales were favoring capelin over pollock as a prey source.     

Behaviour of a sharptail mola in the Gulf of Mexico

A sharptail mola Masturus lanceolatus was tagged and released with a pop-up satellite archival tag in the Gulf of Mexico and tracked for 61 days. The fish travelled a horizontal distance of 594 km while generally staying in water <200 m depth and >20° C. Occasionally the sharptail mola dived to depths >700 m and experienced water colder than 6° C.     

Women from Venus,men from Mars: inter‐sex foraging differences in the imperial cormorant Phalacrocorax atriceps a colonial seabird

Colonial seabirds are central place foragers and likely to be subject to substantial competition for resources. Mechanisms proposed for reducing intra‐specific competition include differential inter‐sex area use mediated by adult choice. We used GPS loggers and dive recorders to study area use and dive depth in a total of 27 male and 26 female imperial cormorants Phalacrocorax atriceps breeding at a colony of some 6500 birds at Punta Leon, Chubut, Argentina during 2004 and 2005. Although time spent travelling and distances between the colony and foraging sites were similar for both sexes, males and females travelled away from their colony using routes virtually perpendicular to each other so that their foraging areas were distinctly different; females hunted close to the coast while males foraged offshore in deeper water. Consideration of foraging efficiency underwater, defined as the duration spent on the bottom divided by the dive cycle duration, showed that females were more efficient at depths < 40 m while males more efficient at depths > 40 m. We suggest that the substantial sexual dimorphism in this species may be responsible for the different depth‐linked foraging efficiencies and that selection for appropriate depths could lead to differential habitat use and putative differences in prey selection.     

Bump-head sunfish Mola alexandrini photographed in the north-west Pacific Ocean mesopelagic zone

We report a bump-head sunfish Mola alexandrini (120–130 cm estimated total length) photographed from a manned submersible at a depth of 220 m off Kagoshima Prefecture, Japan, in May 2004. Mola alexandrini is often misidentified as the ocean sunfish Mola mola. This individual represented three records for this species: first record by a manned submersible, second record from the mesopelagic zone in the north-west Pacific Ocean and a new distributional record from Kagoshima Prefecture, Japan.     

Foraging ecology of Gentoo Penguins Pygoscelis papua at Macquarie Island during the period of chick care

The diet, diving behaviour, swimming velocity and foraging range of Gentoo Penguins Pygoscelis papua were studied at Macquarie Island during the breeding season in the 1993–1994 austral summer. Gentoo Penguins are considered to be inshore feeders, and at Macquarie Island the diet and estimated foraging ranges supported this. The diet consisted of 91.6% fish and 8.3% squid, by mass. The dominant prey taxa were the fish Gymnoscopelus sp. and Paranotothenia magellanica. A mixture of pelagic and benthic prey was consumed, with a greater proportion of benthic species occurring later in the season. The penguins exhibited a strong diurnal pattern in their diving behaviour. Deep diving (≥30 m) began near sunrise (03.00 h) and finished close to sunset (21.00 h). Diving at night was less common and very shallow (<10 m). Early in the breeding season, dive profiles indicated that birds were probably following vertically migrating pelagic prey through the water column and were foraging in waters over 100 m deep. Later in the season, more uniform, shallower depths were used, suggesting an increase in benthic foraging activity. These changes in dive pattern and depth were consistent with the habitat preferences of prey species found in the diet. Gentoo Penguins swam at 1.04 m per s and had a maximum potential foraging range of about 26 km for single-day trips. They tended to forage within 14 km of the colony, with a mean range of 5.4 km. This range encompassed the deep ocean habitat to the west and east of the island and a shallow area to the north.     

Shallow food for deep divers: Dynamic foraging behavior of male sperm whales in a high latitude habitat

Groups of female and immature sperm whales live at low latitudes and show a stereotypical diving and foraging behavior with dives lasting about 45 min to depths of between 400 and 1200 m. In comparison, physically mature male sperm whales migrate to high latitudes where little is known about their foraging behavior and ecology. Here we use acoustic recording tags to study the diving and acoustic behavior of male sperm whales foraging off northern Norway. Sixty-five hours of tag data provide detailed information about the movements and sound repertoire of four male sperm whales performing 83 dives lasting between 6 and 60 min. Dives ranged in depth between 14 and 1860 m, with a median depth of 175 m, and 92% of the surfacings lasted less than 15 min. The four whales clicked for an average 91% (SD = 10) of the dive duration, where the first usual click was produced at depths ranging between 4 and 218 m and the last usual click at depths ranging between 1 and 1114 m. Echolocation buzzes, which are used as an indication of prey capture attempts, were emitted at depths between 17 and 1860 m, during both the descent and ascent phase of deep dives. The foraging behavior varied markedly with depth, with the timing and duration of prey capture attempts during shallow dives suggesting that the whales target more sparsely distributed prey. In contrast, deep dives involve frequent prey capture attempts and seem to target more dense food layers. The evidence of exploitation of different food layers, including epipelagic prey, is consistent with the hypothesis that male sperm whales may migrate to high latitudes to access a productive, multi-layered foraging habitat.     

Species Composition and Barotolerance of Gut Microflora of Deep-Sea Benthic Macrofauna Collected at Various Depths in the Atlantic Ocean

The bacterial flora of marine animals collected at depths of 570 to 2,446 m was examined for population size and generic composition, and the barotolerant characteristics of selected bacterial isolates were determined. Total numbers of culturable, aerobic, heterotrophic bacteria were found to be low in animals collected at the greatest ocean depths sampled in this study. Vibrio spp. were predominant in 10 of 15 samples examined, and Photobacterium spp. and yeasts were the major components of the remainder. Pseudomonas, Achromobacter, and Flavobacterium spp. comprised minor components of the gut flora of deep-sea fish. Forty-six pure cultures isolated from samples of seven animals were tested for growth or viability after incubation for 1 week under pressures ranging from 100 to 750 atm. Strains of bacteria isolated from samples of fish intestine were more barotolerant than those from the stomach (P<0.01). When incubated at a pressure of 600 atm, viability of bacterial cultures originally isolated from fish caught at a depth of 570 m was significantly decreased in comparison with viability of cultures from animals caught at depths of 1,393 and 2,446 m (P<0.01). From results of this study, it is concluded that the gut microflora of animals that dwell in the deeper regions of the ocean are adapted to an increased hydrostatic pressure environment, that is, the gut microflora is less inhibited by elevated hydrostatic pressure with increasing depth from which the host animal was collected.     

Movements of salmon Salmo salar (L.) to and from Irish waters

Fish tagged as kelts at a number of Irish stations have been recaptured outside Irish waters along the coasts of Great Britain, Norway and West Greenland. Salmon tagged as smolts in three Irish rivers have been recaptured off the coast of West Greenland and one off the Faroes. Sixteen fish tagged as smolts in Denmark, Great Britain and Sweden were recaptured in Irish rivers or on the coastline of Ireland. Clean salmon tagged in the open sea have travelled to the coasts of Great Britain and a single fish travelled to southern Sweden. Feeding salmon tagged off West Greenland, in the Labrador sea and off the Faroes have been recaptured in Irish waters.     

Day and night mesopelagic fish assemblages off the Kerguelen Islands (Southern Ocean)

Mesopelagic fish assemblages were investigated in the Polar Frontal Zone off the Kerguelen Islands during summer 1995, in parallel with a king penguin tracking study. During the day, the upper offshore water layers (0–200 m) have low potential prey diversity and abundance with only three fish species: a lanternfish, Krefftichthys anderssoni, a member of the Muraenolepididae, Muraenolepis marmoratus, and the early stages of the nototheniid, Lepidonotothen squamifrons. The mesopelagic fish community, including the typical myctophids, first appears in the deeper layer (300 m). At night, the surface layer (50 m) is invaded by the mesopelagic Myctophidae Electrona antarctica, Gymnoscopelus braueri, G. piabilis, G. fraseri, G. nicholsi, Protomyctophum bolini and P. tenisoni. Deeper (>100 m), a cline of species assemblages from the coast to offshore is observed. Accepted: 4 August 1999     

The influence of spawning and nursery grounds on the distribution of sole Solea solea (L.) in the Irish Sea, Bristol Channel and adjacent areas

In the coastal waters of west England and Wales, sole, Solea solea (L.), are found in greatest abundance in the north-east Irish Sea and the Bristol Channel. Data from research and charter vessel surveys in these areas showed differences in the spatial distribution of age-groups between the two regions. Juvenile sole (0-group, 1-group and 2-group) were found almost exclusively in the shallow (<20 m deep) parts of the north-east Irish Sea and adults (3-group and older) were also found in the same area and in deeper water. In contrast, while juvenile sole in the Bristol Channel were abundant in and around Carmarthen Bay, Swansea Bay and the Severn estuary, they also occurred frequently in deeper areas of the Bristol Channel down to depths of 40 m. Adult sole in the Bristol Channel were also relatively more abundant at depths >40 m than adults in the Irish Sea. The movements of sole which produce these patterns of distribution in the two areas are also shown by the recaptures of sole tagged in the respective nursery areas. The differences in the distribution of sole in the two areas may be related to the depths of their spawning grounds and the distances that maturing fish must travel to reach them.

In the north-east Irish Sea, sole spawning grounds are generally in waters of <40 m depth and within an extensive area of relatively shallow and gently shelving sediments close to the coastal nursery grounds. The bottom topography of the Bristol Channel is more steeply shelving and the spawning grounds of sole are at a depth of 40–75 m off Trevose Head, ≈100 km from the main nursery areas. The hydrographic conditions in the two areas are described, and it is concluded that one of the more important requirements of a successful spawning ground is the presence of suitable hydrographic conditions to transport eggs and larvae to nursery areas.     

Long-Term GPS Tracking of Ocean Sunfish Mola mola Offers a New Direction in Fish Monitoring

Satellite tracking of large pelagic fish provides insights on free-ranging behaviour, distributions and population structuring. Up to now, such fish have been tracked remotely using two principal methods: direct positioning of transmitters by Argos polar-orbiting satellites, and satellite relay of tag-derived light-level data for post hoc track reconstruction. Error fields associated with positions determined by these methods range from hundreds of metres to hundreds of kilometres. However, low spatial accuracy of tracks masks important details, such as foraging patterns. Here we use a fast-acquisition global positioning system (Fastloc GPS) tag with remote data retrieval to track long-term movements, in near real time and position accuracy of <70 m, of the world''s largest bony fish, the ocean sunfish Mola mola. Search-like movements occurred over at least three distinct spatial scales. At fine scales, sunfish spent longer in highly localised areas with faster, straighter excursions between them. These ‘stopovers’ during long-distance movement appear consistent with finding and exploiting food patches. This demonstrates the feasibility of GPS tagging to provide tracks of unparalleled accuracy for monitoring movements of large pelagic fish, and with nearly four times as many locations obtained by the GPS tag than by a conventional Argos transmitter. The results signal the potential of GPS-tagged pelagic fish that surface regularly to be detectors of resource ‘hotspots’ in the blue ocean and provides a new capability for understanding large pelagic fish behaviour and habitat use that is relevant to ocean management and species conservation.     

Living benthic foraminifera of the Okhotsk Sea: Faunal composition, standing stocks and microhabitats

Live (Rose Bengal stained) benthic foraminifera were investigated in surface sediment samples from the Okhotsk Sea to reveal the relationship between faunal characteristics and environmental parameters. Live benthic foraminifera were quantified in the size fraction > 125 µm in the upper 8 cm of replicate sediment cores, recovered with a multicorer at five stations along the Sakhalin margin, and at three stations on the southwestern Kamchatka slope. The stations are from water depths between 625 to 1752 m, located close or within the present Okhotsk Sea oxygen minimum zone, with oxygen levels between 0.3 and 1.5 ml l^- 1. At the high-productivity and ice-free Kamchatka stations, live benthic foraminifera are characterized by maximal standing stocks (about 1700-3700 individuals per 50 cm²), strong dominance of calcareous species (up to 87-91% of total live faunas), and maximal habitat depths (down to 5.2-6.7 cm depth). Vertical distributions of total faunal abundances exhibit a clear subsurface maximum in sediments. At the Sakhalin stations, which are seasonally ice-covered and less productive, live benthic foraminifera show lower standing stocks (about 200-1100 individuals per 50 cm²), lower abundance of calcareous species (10-64% of total live faunas), and shallower habitat depths (down to 2.5-5.4 cm depth). Faunal vertical distributions are characterized by maximum in the uppermost surface sediments. It is suggested that 1) lower and strongly seasonal organic matter flux, caused by the seasonal sea ice cover and seasonal upwelling, 2) lower bottom water oxygenation (0.3-1.1 ml l^- 1), and 3) more pronounced influence of carbonate undersaturated bottom water along the Sakhalin margin are the main factors responsible for the observed faunal differences. According to species downcore distributions and average living depths, common calcareous species were identified as preferentially shallow, intermediate and deep infaunal. Foraminiferal microhabitat occupation correlates with the organic matter flux and the depth of the oxygenated layer in sediments.     

Movements and dive behaviour of two leopard seals (<Emphasis Type="Italic">Hydrurga leptonyx</Emphasis>) off Queen Maud Land,Antarctica

Two adult female leopard seals (Hydrurga leptonyx) were tagged with satellite-linked dive recorders off Queen Maud Land, Antarctica, just after moulting in mid-February. The transmitters transmitted for 80 and 220 days, respectively. Both seals remained within the pack ice relatively close to the Antarctic Continent until early May, when contact was lost with one seal. The one remaining seal then migrated north, to the east side of the South Sandwich Islands in 3 weeks, whereafter it headed east, until contact was lost at 55°S in early September. From mid-May to late September this animal always stayed close to the edge of the pack ice. Both seals made mostly short (<5 min) dives to depths of 10–50 m and only occasionally dove deeper than 200 m, the deepest dive recorded being 304 m. A nocturnal diving pattern was evident in autumn and early winter, while day-time diving prevailed in mid-winter. Haul out probability was highest at mid-day (about 40% in late February and more than 80% in March and April). From May till September the remaining animal mainly stayed at sea, in the vicinity of the pack ice, with only occasional haul outs. These data suggest that a portion of the adult leopard seals may spend the winter mainly in open water, off the edge of the pack ice, where they primarily hunt near the surface. In that case, it is likely that krill (Euphausia superba), as well as penguins, young crabeater seals (Lobodon carcinophaga) and a variety of fish are important prey items.